1. Field of the Invention
The present invention relates to a mounting structure for a superconductor device, and more particularly to a mounting structure for a high temperature superconductor device housed in a closed vacuum chamber and operated at a low temperature.
2. Description of the Related Art
There have so far been proposed a wide variety of superconductor devices, especially high temperature superconductor (hereinlater referred to simply as "HTS") devices, preferably utilized for an integrated circuit, a filter, an amplifier and so forth. This type of superconductor device generally comprises a dielectric substrate and superconducting thin film layers deposited on both of surfaces of the dielectric substrate by a physical vapor deposition method, e.g., a sputtering method or a reactive vapor deposition method. Each of the superconducting thin film layers is made of a ceramics system, such as an yttrium, barium, and copper oxide system (hereinlater referred to simply as a "YBaCuO system") of the HTS.
The superconductor has an extremely low electric resistance below a critical temperature thereby causing superconductive phenomena. The superconductor has therefore an advantage over a normal metal conductor in reducing transmission loss of signals.
Typical superconductor device is however required to be housed in a closed vacuum chamber at a low pressure of 2.times.10.sup.-2 Pa or less and cooled at a low temperature of 80 K in order to cause the aforesaid superconductive phenomena. The superconductor device should therefore be contained in a device holding apparatus 1 as shown in FIG. 13 to keep the above pressure and temperature.
As shown in FIG. 13, the device holding apparatus 1 comprises a housing 2 formed with a closed vacuum chamber and a cooling device 3. The cooling device 3 has a cold head 3a for placing the superconductor device 4 thereon and a cold finger 3b for holding the cold head 3a at a predetermined low temperature to cool the superconductor device 4. The superconductor device 4 and the cooling device 3 are housed in the closed vacuum chamber. The cold finger 3b may comprise a coolant reservoir (not shown) for reserving a liquid helium or a liquid nitrogen, or another type of cooling device (not shown), such as a Stirling cycle cooler or a pulse tube type of cooler.
The device holding apparatus 1 further comprises an input connector 5 electrically connected to the superconductor device 4 through a signal inputting line 5a and an output connector 6 electrically connected to the superconductor device 4 through a signal outputting line 6a. The input and output connectors 5 and 6 are further electrically connected to external devices (not shown) outside of the device holding apparatus 1. The superconductor device 4 can thus transmit a signal from and to the external devices through the input and output connectors 5 and 6.
Referring to FIGS. 14 and 15 of the drawings, there is shown a conventional mounting structure 90 for the superconductor device 4 shown in FIG. 13. In this example, the superconductor device 4 is a planer band-pass filter 91. The filter 91 is adapted to have a signal inputted from a first external device (not shown in the drawings) to output a second external device (not shown).
In this example, the filter 91 comprises a dielectric substrate 92 having first and second surfaces 92a and 92b diametrically opposite to each other. The first surface 92a of the dielectric substrate 92 is shown in FIG. 15 as being an upper side surface, while the second surface 92b of the dielectric substrate 92 is shown in FIG. 15 as being a lower side surface. The dielectric substrate 92 is made of a MgO.
The filter 91 further comprises a circuit layer 93 having a pattern of circuit lines made of a superconducting thin film and deposited on the first surface 92a of the dielectric substrate 92. The superconducting thin film is made of a ceramics system, such as a YBaCuO system, of the HTS.
The filter 91 comprises a ground layer, not shown in the drawings, consisting of a superconducting thin film layer and a metal layer. The superconducting thin film layer of the ground layer is made of a YBaCuO system of the HTS and deposited on the second surface 92b of the dielectric substrate 92. The metal layer of the ground layer is deposited on the superconducting thin film layer of the ground layer.
The conventional mounting structure 90 as shown in FIGS. 14 and 15 comprises a device holder 94, a plurality of fastening parts 95, input and output connectors 96a and 96b, and an adhesive layer 97.
The device holder 94 has a base surface 94a and is adapted to hold the filter 91 thereon. The device holder 94 is grounded where the base surface 94a of the device holder 94 and the ground layer of the filter 91 are electrically connected with each other. As shown in FIG. 15, the base surface 94a is formed on an upper side surface of the device holder 94 into a smoothed flat plane. The base surface 94a of the device holder 94 is made of a conductive material selected from among the group consisting of copper and aluminum and covered with a nickel and gold.
The input connector 96a is electrically connected to the filter 91 and the first external device through the input connector 5 shown in FIG. 13 to allow the signal to be inputted from the first external device to the filter 91. The output connector 96b is electrically connected to the filter 91 and the second external device through the output connector 6 shown in FIG. 13 to allow the signal to be outputted from the filter 91 to the second external device. The filter 91 can thus transmit the signal from and to the first and second external devices outside of the device holding apparatus 1.
The adhesive layer 97 intervenes between the ground layer of the filter 91 and the device holder 94. The adhesive layer 97 has a first surface 97a facing the ground layer of the filter 91 and a second surface 97b facing the base surface 94a of the device holder 94. In this example, the adhesive layer 97 is made of an indium foil covering a whole area of the ground layer of the filter 91 therewith.
The fastening parts 95 are operated to fasten the filter 91 on the device holder 94, so that the first surface 97a of the adhesive layer 97 can be held in contact with the ground layer of the filter 91 and the second surface 97b of the adhesive layer 97 can be also held in contact with the base surface 94a of the device holder 94. Each of the fastening parts 95 includes a pressing member 95a and a clamp screw 95b screwed into the device holder 94 to secure the pressing member 95a to the device holder 94.
As shown in FIG. 14, the filter 91 has a circuit portion on which there is the circuit layer 93 and a peripheral portion on which there is no circuit layer. In this example, the fastening parts 95 are arranged along the peripheral portion of the device holder 94 at eight points to secure the filter 91 at its peripheral portion on the device holder 94 as shown in FIG. 14.
Two of the eight points are especially positioned at the places adjacent to the input and output connectors 96a and 96b in order to ensure that the input and output of the filter 91 are securely grounded. Other than the above two points are spaced apart from each other at predetermined intervals in order to prevent the signal in a high frequency from leaking from the ground layer of the filter 91. From this point of view, each of intervals of these positions may be assumed to be equal to or less than a half wavelength .lambda./2 of the band-pass frequency of the filter 91. In this example, the filter 91 has the half wavelength .lambda./2 of 150 mm as the band-pass frequency is about 1 GHz. Therefore, the fastening parts 95 may be spaced apart from each other at the intervals of 150 mm or less.
Referring to FIG. 16 of the drawings, there is shown a graph showing a filter function in a frequency response of a typical filter including the above filter 91. As shown in FIG. 16, the typical filter has a large response within a passband. The frequency response of the filter is attenuated outside of the passband, more specifically in a frequency region outside of a region between f.sub.L and f.sub.H as shown in FIG. 16. The typical filter has a filter function in frequency response generally defined as attenuation "A" outside of the passband of 90 dB or more. Likewise, the HTS device may preferably have a filter function in frequency response defined as the attenuation A outside of the passband of 90 dB or more.
In the conventional mounting structure 90, the adhesive layer 97 can be made of an indium, which is inexpensive. The indium has a specific resistance .rho. of 8.8 .mu..OMEGA..multidot.cm and a modulus of elasticity of 1.57.times.10.sup.6 psi.
The above adhesive layer 97 is, however, liable to seal a gas in a boundary between the first surface 97b of the adhesive layer 97 and the ground layer of the filter 91 and a boundary between the second surface 97b of the adhesive layer 97 and the base surface 94a of the device holder 94, owing to the extremely low modulus of elasticity of the indium. The sealed gas in the aforesaid boundaries is gradually released, thereby making it impossible to keep the specific low pressure of 2.times.10.sup.-2 Pa in the closed vacuum chamber and the specific low temperature of 80 K.
In order to solve the above problem in the conventional mounting structure 90, the closed vacuum chamber is conventionally being pumped down to a low pressure of lower than 2.times.10.sup.-2 Pa by a vacuum pump (not shown) while the device 4 is being operated in the closed vacuum chamber. Therefore, the gas can be pumped out even when the gas sealed in the boundaries is released while the device is operated in the closed vacuum chamber.
Recently, the device holding apparatus 1 must be reduced in size. Moreover, the vacuum pump must be removed after housing the superconductor device 4 and pre-pumping the gas out from the device holding apparatus 1. It is impossible to pump down the closed vacuum chamber while superconductor device 4 is being operated. Therefore, the sealed gas cannot be pumped out even when the sealed gas is released from the boundaries to the vacuum chamber.
In order to keep a low pressure of 2.times.10.sup.-2 Pa, the amount of the gas released from the boundaries should be limited to 1.times.10.sup.-9 Pa.multidot.m.sup.3 /sec or less, which is the same amount of the gas released from a chamber wall of the housing 2. The gas sealed in the boundaries is, however, apt to be gradually released to have the amount of the released gas exceed this limitation of 1.times.10.sup.-9 Pa.multidot.m.sup.3 /sec, thereby making it impossible to keep the specific low pressure of 2.times.10.sup.-2 Pa while the device is being operated in the conventional mounting structure 90. This causes a problem for the operation of the superconductor device.
Furthermore, the superconductor device such as a filter is required to have a high conductivity because the ground layer of the superconductor device has an extremely low contact resistance against the base surface 94a of the device holder 94. The conventional mounting structure 90 however cannot establish a high conductivity because the adhesive layer 97 is made of the indium which has a low conductivity, i.e., the specific resistance .rho. of 8.8 .mu..OMEGA..multidot.cm.